The solar system is densely packed with planets and also contains an asteroid and a Kuiper belts, remnants from the planet-formation epoch. Are planetary systems with high-mass planets any different in terms of remnant planetesimal belts from those with low-mass planets or those with no known planets? What does this tell us in terms of planetary system formation and evolution?

Image credit: Lynette Cook

Planetesimals are the building blocks of planets, and mid and far-infrared observations with Spitzer and Herschel indicate that at least 10–25% of mature stars (10 Myr to 10 Gyr) harbor planetesimal disks with disk sizes of tens to hundreds AU (this frequency is a lower limit because the surveys are limited by sensitivity). The evidence for planetesimals comes from the presence of circumstellar dust: because the lifetime of the dust grains (<1 Myr) is much shorter than the age of the star ( >10 Myr), it is inferred that the dust cannot be primordial but must be the result of steady or stochastic dust production generated by the collision, disruption, and/or sublimation of planetesimals, like the asteroids, comets and Kuiper belt objects in our solar system. The presence of these debris disks in both single- and multiple-star systems, and around A- to M-type stars (also around the progenitors of white dwarfs), spanning several orders of magnitude difference in stellar luminosities, imply that planetesimal formation, a critical step in planet formation, is a robust process that can take place under a wide range of conditions. It is therefore not surprising that in some cases planets and debris disks coexist. But are dust-producing planetesimal disks more or less common around stars with planets? Using the evolution of the solar system as a model, in its early history, a star with planetary companions could be expected to be surrounded by a massive debris disk produced by the planetesimal swarm that formed the planets, the latter exciting planetesimal collisions and dust-production while undergoing orbital migration. On the other hand, at a later stage, the star could harbor a sparse dust disk after the dynamical rearrangement of the planets is complete and the planetesimal swarm has undergone significant dynamical clearing. Do observations support these trends?

Because the study of the planet-debris disk correlation could shed light on the formation and evolution of planetary systems and may help “predict” the presence of planets around stars with certain disk characteristics, we have carried out a statistical study of an unbiased sub-sample of the Herschel DEBRIS and DUNES debris disk surveys, to assess whether the frequency and properties of debris disks around a control sample of solar-type stars are statistically different from those around stars with planets. Out of the 466 and 133 stars in the DEBRIS and DUNES samples, respectively, we have selected a subsample of 204 FGK stars located at distances <20 pc (to maximize survey completeness), with ages >100 Myr (to avoid introducing a bias due to disk evolution), and with no binary companions at <100 AU (to avoid introducing a bias due to the observed differences in both disk frequency and planet frequency between singles and multiples). The debris-disk frequency within this unbiased sample is 0.14 +0.3/-0.2 .

In this clean sample, we don’t find any evidence that debris disks are more common or more dusty around stars harboring high-mass planets (> 30 MEarth) compared to the average population. Overall, this lack of correlation can be understood within the context that the conditions to form debris disks are more easily met than the conditions to form high-mass planets, in which case one would not expect a correlation based on formation conditions; this is also consistent with the studies that show that there is a correlation between stellar metallicity and the presence of massive planets, but there is no correlation between stellar metallicity and the presence of debris disks. Another factor contributing to the lack of a well-defined correlation might be that the dynamical histories likely vary from system to system, and stochastic effects need also to be taken into account, e.g., those produced by dynamical instabilities of multiple-planet systems clearing the outer planetesimal belt or the planetesimal belt itself triggering planet migration and instabilities.

Regarding low-mass planets (< 30 MEarth), one would expect that if the planets formed in the outer region and migrated inward, low-mass planets would have been inefficient at accreting or ejecting planetesimals, leaving them on dynamically stable orbits over longer timescales. On the other hand, high-mass planets would have been more efficient at ejecting planetesimals, leaving behind a depleted population of dust-producing parent bodies. Alternatively, if the planets formed in situ, the timescale for the planet to eject the planetesimals would have been shorter in systems with high-mass planets than with low-mass planets. Under both scenarios, from an evolution point of view, one would expect to find a positive correlation between low-mass planets and the presence of a remnant dust-producing planetesimal disk and, in fact, preliminary analyses of the Herschel surveys have found tentative evidence of such correlation. However, our clean sample does not confirm the presence of this correlation. Why? It could be because the true migration histories of the systems studied may be significantly more complicated than the two scenarios described above; for example, in our own solar system, it is now well established that the ice giants, Uranus and Neptune, migrated outward over a significant distance to reach their current locations, sculpting the trans-Neptunian population as they did so. Another explanation could be because the planets detected by radial velocity surveys and the dust observed at 100 μm occupy well-separated regions of space, limiting the influence of the observed closer-in planets on the dust production rate of the outer planetesimal belt. But it could also be that our sample is too small to detect such a correlation because having a clean sample that avoids the biases mentioned above comes at a price: in our sample, a positive detection of a correlation could have been detected only if the disk frequency around low-mass planet stars were to be about four times higher than the control sample.

Another aspect that we have explored is the role of planet multiplicity. Dynamical simulations of multiple-planet systems with outer planetesimal belts indicate that there might be a correlation between the presence of multiple planets and debris. This is because the presence of the former indicates a dynamically stable environment where dust producing planetesimals may have survived for extended periods of time (as opposed to single-planet systems that in the past may have experienced gravitational scattering events that resulted in the ejection of other planets and dust-producing planetesimals). However, our sample does not show evidence that debris disks are more or less common, or more or less dusty, around stars harboring multiple-planet systems compared to single-planet systems.

And how do the observed debris disks compared to our solar system? Because our sample does not show any evidence of disk evolution in Gyr timescales, we can look at the distribution of disk fractional luminosities (Ldust/Lstar; a distance-independent variable). We find that a Gaussian distribution of fractional luminosities in logarithmic scale centered on the solar system value (taken as 10-6.5) fits the data well, whereas one centered at 10 times the solar system’s debris disks can be rejected. This is of interest in the context of future prospects for terrestrial planet detection. Even though the Herschel observations presented in this study trace cold dust located at tens of AU from the star, for systems with dust at the solar system level, the dust dynamics is dominated by Poynting–Robertson drag. This force makes the dust in the outer system drift into the terrestrial-planet region. This warm dust can impede the future detection of terrestrial planets due to the contaminant exozodiacal emission. Ruling out a distribution of fractional luminosities centered at 10 times the solar system level implies that there are a large number of debris disk systems with dust levels in the KB region low enough not to become a significant source of contaminant exozodiacal emission. Comets and asteroids located closer to the star are other sources of dust that can contribute to the exozodiacal emission (and for those, Herschel observations do not provide constraints), but planetary systems with low KB dust-type of emission likely imply low-populated outer belts leading to low cometary activity. These results, therefore, indicate that there are good prospects for finding a large number of debris disk systems (i.e., systems with evidence of harboring planetesimals) with exozodiacal emission low enough to be appropriate targets for terrestrial planet searches.

Larger samples are needed to improve the statistics of the studies mentioned above, but, as we have done here, care must be taken to avoid biases. But increasing the sample size is not enough. There are two additional aspects that need to be improved upon and, with the data at hand, cannot be addressed at the moment: our ability to detect fainter debris disks (as we may only have detections for the top 20% of the dust distribution), and to detect or rule out the presence of lower-mass planets to greater distances. For the later, of critical importance is that the planet search teams make the non-detections publicly available so we can identify systems for which the presence of planets of a given mass can be excluded out to a certain distance.

Antoine de Saint-Exupéry, the world-famous writer of The Little Prince, served as an aviator for the French Aéropostale over the Paris-Dakar route, crossing the African desert for many years in the 1930s. In his book Wind, Sand and Stars he reports of having once perceived the forthcoming conflagration of a sand storm before taking off from an intermediate station in the Sahara, by observing the peculiar behavior of two dragonflies. “What filled me with joy was that I had understood a murmured monosyllable of this secret language, that I had been able to read the anger of the desert in the beating wings of a dragonfly.” This joy is not unknown to the astrophysicist. Here is a story.

Over the last year, I happened to be working on a survey of infrared emission spectra of carbon monoxide (CO) observed in young “protoplanetary” disks, the birthplaces of the plethora of exoplanets detected so far. The CO molecule is generally abundant in planet-forming regions, at disk radii comprised to within approximately 10 Astronomical Units (AU) from the central star. CO had been observed in disks for over thirty years [1], and recent instrumental developments had made possible to perform a survey of unprecedented sensitivity, spectral resolution, and sample size in the years 2007-2010 [2]. While studying the peculiar flickering behavior of CO and water emission from the disk of a variable star, I noticed that the CO spectra looked like the superposition of two emission line components, one being distinctly broader than the other [3]. I attempted a spectral decomposition analysis, encouraged by the exquisite quality of the data, and found that while many protoplanetary disks showed both CO components, some had only the narrow one [4]. By measuring the temperature (from the line flux ratios) and the disk radius of CO emission (from the line widths) in each disk of the survey, I composed the diagram shown below. When I and Klaus Pontoppidan, my collaborator and mentor, looked at it, we were astonished by the appearing of a sequence.

Figure (click to enlarge): The temperature-radius (T-R) diagram of rovibrational CO emission in disks [4]. The red and blue data points are individual disks from high quality, high spectral resolution surveys done with CRIRES at the VLT (resolving power of ~100,000) [2,5]. The sample spans a range in stellar masses of 0.5-3 solar masses (indicated by the symbol size). The location of each disk in the diagram indicates the vibrational temperature of the innermost CO gas present in the disk. At the bottom of the figure, for comparison, are shown the Solar System planets, together with the distribution of semi-major axes of observed exoplanets with Msini > 0.5 Jupiter masses [6].

Given its high dissociation temperature, CO traces the innermost disk radius where molecular gas can survive in any disk. Therefore, the location of each disk in the diagram indicates the temperature of the innermost molecular gas present in its planet-forming region. The red disks in the diagram are those found to have two CO components and are identified as “primordial”, where the inner radius is set by the stellar magnetospheric accretion or by dust sublimation (truncating the disk out to ~0.1 AU at most for the whole sample). Blue disks lack the broad CO component, and have something else going on preventing CO gas from extending all the way to the smallest distance allowed by the stellar properties…

As the CO emission analyzed here is rovibrational, the measured line ratios give a vibrational temperature, which is a sensitive thermometer of the local radiation field. The temperature-radius (T-R) diagram, taken as a whole, reveals a sequence composed of two regimes. In the inner 0.03-2 AU the temperature decreases as a power-law profile, as expected for the dust temperature in models of inner disks irradiated by the central star. This regime is identified as due to infrared pumping of CO by the local warm dust, and provides an empirical temperature profile for inner disks around solar-mass stars. The second regime takes over beyond ~2 AU, and shows an inversion in the temperature. This temperature inversion strongly points at another excitation mechanism that is known to effectively populate CO lines in low-density and cold environments: ultraviolet (UV) fluorescence [7]. In order for UV radiation to be effective at such large distances from the central star (2-20 AU), the innermost region of these disks must be largely depleted in both dust grains and molecular gas. These disks must host large inner gaps in their radial structure. Overall, CO emission suggests that all blue disks are developing or have developed large inner gaps, and some of them (filled symbols in the figure) have already been identified as “transitional” by dust emission modeling or by direct imaging. The T-R diagram has the power to provide prime targets for direct imaging campaigns, pushing inward the detection of inner gaps to radii that will become accessible to future infrared imagers (e.g. by E-ELT-METIS [8]).

But the best is yet to come. This research provides an empirical framework to investigate gap-opening processes in disks, including planet formation and migration. Comparison of the CO temperature sequence to the distribution of giant exoplanets detected so far reveals two interesting facts. The so-called “hot”-Jupiters are found at the innermost radial location of CO gas in disks, ensuring that abundant gas is present to allow gas-supported planet migration as proposed by models [9]. The distribution of exo-Jupiters, instead, rises at the break point between the two regimes in the CO diagram, supporting the existence of a link between exo-Jupiters formation and the opening of gaps in the natal disks [10], which eventually leads to their dispersion through the “debris disk” phase [11]. The journey of an exoplanet from its birth is long and can be full of surprises, ending up in the large diversity suggested by the foremost research in planetary architectures and compositions [e.g. 12]. And for us, at the horizon, stands the possibility of finding something similar to what we know here on Earth, a journey that is breathtaking for our entire world. It may still be far ahead in time, but every word we catch of this secret story of nature is welcomed with joy by those who spend their lives aspiring to hear it in full. Sometimes, these words are found in the most unexpected data, or in a diagram composed almost by chance. Sometimes, we can understand a murmured monosyllable of this secret language simply by “following the wings of a dragonfly”.

Debris disks are cold dust belts hosted by some main-sequence stars, composed of micrometer-size grains to kilometer-size planetesimals. As left-overs from planet formation, the study of these young cousins of our own solar system’s Kuiper belt can help us to better understand how planets are formed.

To do so, we need to image these disks in the visible or near-infrared, to deduce their composition and physical properties from the starlight scattered by the dust, and maybe also detect signposts of planets in their geometry. However, despite numerous surveys with the Hubble Space Telescope (HST) and the largest ground-based telescopes, only 19 debris disks had been imaged in scattered light so far. As far as planets are concerned, only 24 have been directly imaged, in significant contrast with the large numbers of discoveries by the radial velocity and the transit methods (with hundreds and thousands of planet detections respectively).

This is due to the very high contrast between the host star and the light reflected by a disk or emitted by a planet (more than a million times fainter!). To achieve such challenging detections, the instruments need to be equipped with carefully optimized coronagraphs, and with efficient adaptive optics systems for ground-based telescopes, and furthermore, the observer has to apply post-processing techniques on the resulting images to detect the dim circumstellar material.

The classical post-processing method consists of subtracting the image of a reference star from the science image to reveal material in its vicinity. However, such a subtraction is never perfect due to telescope instabilities and/or residual wavefront errors, and residual starlight still impedes the detection of cold material within 2’’ of the star (Fig. 1). New algorithms have been recently developed to solve this issue, by using large libraries of reference star images to generate a synthetic image of the star optimized to the actual science image. These new techniques improve the starlight subtraction by a factor of 10 to 100 over the classical method (Fig. 2).

Figure 1: The Principle of the classical post-processing technique: the image of a reference star is subtracted from the science image to remove the starlight. Although this method improves the contrast by a factor of 5 to 10 compared to the raw image, the telescope instabilities prevent the detection of any material within 2’’ from the star.

Figure 2: Images of the debris disk around HD181327 reduced with the classical technique (left, from [1]) and with the KLIP algorithm [2] (right, from [3]). This advanced post-processing algorithm typically improves the contrast by a factor of 10 to 100 over the classical method.

Our team has thus started the project of reprocessing the entire HST-NICMOS coronagraphic archive with such advanced algorithms to reveal new disks and planet candidates [4]. The archive is composed of images of 400 stars observed in the near-infrared between 1997 and 2008 and have been underexploited by the use of mainly old post-processing techniques. Among our recent discoveries from this project is the detection of five debris disks seen for the first time in scattered light (Fig. 3). These detections increase the total number by more than 20%. The on-going analysis and modeling of these disks should tell us more about their composition and properties and maybe present hints of possible planets.

As the number of confirmed (exo)planets grows rapidly, infrared spectroscopy is providing an exceptional opportunity to study the molecular environments where planets are being formed. The Spitzer Space Telescope recently revealed a dense forest of emission lines from water, OH, and some organic molecules (Figure 1) tracing warm/hot gas (200 < T < 1000 K) in young protoplanetary disks inward of the water snow line (the condensation/evaporation boundary between gas and ice in the disk)1. This “steam” emission offers a unique observational link to a variety of processes ongoing inside planet factories.

Figure 1 : Infrared emission from water and other gas molecules in the planet formation region of a young protoplanetary disk (around RNO90, a G5 star), as observed with the Spitzer Space Telescope. All the prominent emission features, apart from those labelled differently, are due to water vapor (a model2 of water emission is superimposed in blue).

Analyzing the rich infrared molecular emission, we recently had the opportunity to investigate its connection to disk evolution and planet formation processes. By comparing infrared spectra of young solar-mass stars, taken at different phases of their accretion of circumstellar material, I and my collaborators found that strong accretion outbursts are able to dramatically affect the molecular content at planet-forming radii in the disk3. An increased heating causes a recession of the snow line to larger disk radii, probably triggering evaporation of water ice, while a harsher UV radiation photodissociates water vapor producing OH (as seen in the infrared spectrum of EX Lupi during a strong recent outburst, see Figure 2). The fate of organic molecules, which disappear during outburst, remains unclear. If accretion outbursts are ubiquitous in star formation, the evolution of material in the planet formation region may be strongly linked to the accretion histories, probably affecting also the chemical/physical properties of forming solid bodies.

Figure 2 : Changes in molecular emission from the disk of EX Lupi during a strong accretion outburst (from Banzatti et al. 2012). Water and OH emission increases, in connection to a recession of the snow line and a stronger UV radiation. The emission from organic molecules instead disappears.

Infrared spectroscopy of planet formation regions also allows us to study the migration of icy solids in the midplane of disks. As icy dust grains stick together in the outer disk, they form larger particles that at some point decouple from the gas and are dragged toward the star. When they reach and cross the snow line, the ice is evaporated providing large abundances of water vapor in the inner disk, unless forming planets accrete them at outer radii4. Evidence for ongoing ice migration is provided by inner disks where the water vapor abundance exceeds the oxygen budget available to form it in situ, while “drier” disks may be advanced in depleting the outer disk from migrators, or have already formed large accreting planetesimals outward of the snow line. In a recent work5, we have shown how a rotation diagram analysis of infrared water vapor emission offers a useful tool to distinguish between these two scenarios, from the spread of the rotational scatter (Figure 3). Such studies of water vapor emission are lifting the veil on processes taking place in disk midplanes that have been until now elusive to our observations.

Figure 3 : Rotation diagram analysis of infrared water vapor emission in protoplanetary disks (from Banzatti et al. 2013). Line opacities are color-coded in blue, while dot sizes are proportional to line intensities. The larger the amount of water vapor in the inner disk, the larger the spread of the rotational scatter in the diagram. The plot to the right shows the case of an inner disk water abundance larger than the oxygen budget available in situ, providing evidence for enrichment from inward icy migrators that evaporate at the snow line.

The number of protoplanetary disks and of molecular emission lines from planet formation regions observed with Spitzer is by far the largest provided by any other telescope to date. This unique dataset has already given the opportunity for pioneering studies of the properties and evolution of the molecular environments during planet formation. Yet, it is likely that we are just scratching the surface. These studies offer us a fertile ground for planning observations with the upcoming James Webb Space Telescope, which promises to narrow the gaps in our understanding of how planets form and to bring us closer to deciphering planet diversity, so to understand better even our own Earth.

This Month’s Featured Author

Dr. Brian Williams received his B.S. from Florida State University in 2004 and his Ph.D. from North Carolina State University in 2010. He was a NASA Postdoctoral Fellow at NASA Goddard Space Flight Center for three years, after which he worked as a research scientist at NASA GSFC with Universities Space Research Association. He arrived at STScI in February of 2017, and is currently a Support Scientist in the Science Mission Office. His research interests include supernovae and supernova remnants, shock physics and particle acceleration, and dust in the interstellar medium.